This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-82583, filed Mar. 23, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a system interconnection protective device for a private generator set such as an urban-waste power generation system and a cogeneration system, which interconnects with a system power supply via a circuit breaker.
FIG. 1 shows a conventional system interconnection protective device that has been used in order that a customer can interconnect a system power supply with a private generator set such as an urban-waste power generation system and a cogeneration system.
As FIG. 1 illustrates, a host substation 4 drops a voltage of a system power supply 1 via a transformer 2 and supplies power to a customer 7 through a circuit breaker 3.
The customer 7 supplies the power to a load 6 through a circuit breaker 5.
The private generator set 10 supplies an output voltage of an alternating-current generator 12 to the system power supply 1 via a circuit breaker 11.
To control the output voltage of the alternating-current generator 12, an automatic voltage regulator (AVR) 13 controls a voltage of a field winding 14 of the generator 12.
To control the output frequency of the generator 12, a governor 15 of an engine 16 for driving the generator 12 controls power of the engine.
A generator fault detection circuit 22 is employed as a failure detection means. The circuit 22 detects an abnormal current based on a relationship between an output current of the generator 12 detected by a current transformer 21 and an output voltage of the generator 12. The circuit 22 then supplies a detection signal of the abnormal current to a trip circuit 23 to thereby open the circuit breaker 11.
A current transformer 24 is provided as a protecting means on the output side (the substation side) of the circuit breaker 11. An overcurrent relay (OC) 25 receives an output of the current transformer 24 to operate the trip circuit 23.
When the system power supply 1 malfunctions, especially when the power supply 1 is broken or the circuit breaker 3 is opened, various types of protective relays, such as an underfrequency relay (UF) 26, an overfrequency relay (OF) 27, an overvoltage relay (OV) 28, and an undervoltage relay (UV) 29, detect that an abnormal frequency or voltage is generated from an imbalance between an output power of the generator 12 and a load power of the load 6.
In response to a detection signal output from the protective relays, the trip circuit 23 gives a trip instruction to the circuit breaker 11 to open the circuit breaker 11 and allow the circuit breaker 3 to be reclosed.
The foregoing system interconnection protective device has the following problems:
If the active and reactive components of the output power of the alternating-current generator 12 and the required power of the load 6 are almost equal to each other when the system power supply 1 malfunctions to open the circuit breaker 3, the frequency and the voltage hardly vary. For this reason, none of the protective relays 25 to 29 operate, but the private generator set continues to operate. A so-called sole-operation (islanding) phenomenon occurs to prevent the circuit breaker 3 to be reclosed.
Some conventional devices adopt the following method in order to prevent the above islanding-operation phenomenon:
In this method, a transfer breaker 8 is connected to a private line extending from the substation 4. This device 8 causes the circuit breaker 11 to break the transfer of power.
In other words, the transfer breaker 8 detects a signal indicating that the circuit breaker 3 of the host substation 4 is opened and transmits a break signal to the circuit breaker 11 to open the breaker 11.
However, the cost of the transfer breaker 8 is very high for the private generator set 10 having a small-to-medium power of several hundreds of kilowatts. There is no practical merit in providing the transfer breaker in the system interconnection protective device.
It is accordingly an object of the present invention to provide a system interconnection protective device for a private generator set, which is capable of reliably and easily detecting and protecting a sole operation of the private generator set, which interconnects with a system, without providing any expensive transfer breaker.
According to a first aspect of the present invention, there is provided a system interconnection protective device for a private generator set, which interconnects with a system power supply via a circuit breaker, the device comprising frequency detection means for detecting a frequency from an output of the private generator set, frequency change rate detection means for detecting a frequency change rate of the frequency detected by the frequency detection means, voltage variation reference determination means for defining a relationship between the frequency change rate and voltage variation reference by a function, the voltage variation reference increasing a leading reactive power of the private generator set when the frequency change rate detected by the frequency change rate detection means is positive increasing a lagging reactive power of the private generator set when the frequency change rate is negative, frequency change rate integration means for integrating the frequency change rate detected by the frequency change rate detection means, frequency variation tendency determination means for determining whether the frequency rises or lowers, based on a frequency change rate integration amount integrated by the frequency change rate integration means, bias means for applying a bias to the voltage variation reference output from the voltage variation reference determination means to decrease the voltage variation reference when the frequency variation tendency determination means determines that the frequency rises and applying a bias to the voltage variation reference output therefrom to increase the voltage variation reference when the frequency variation tendency determination means determines that the frequency lowers, and protection means for detecting a frequency variation promoted due to a voltage variation of the private generator set caused by adding the voltage variation reference to which the bias is applied by the bias means, to voltage reference of an automatic voltage regulator of the private generator set, thereby opening the circuit breaker and disconnecting the private generator set from a system bus.
In addition to the above means, the system interconnection protective device may further comprise reset means for determining whether the frequency change rate detected by the frequency change rate detection means falls within a narrow range close to zero and resetting the frequency change rate integration amount integrated by the frequency change rate integration means to zero when the frequency change rate falls within the small range. The protection means can determine whether the private generator set is to be disconnected from the system line on a first condition that the frequency change rate detected by the frequency change rate detection means exceeds a value, on a second condition that the frequency change rate integration amount integrated by the frequency change rate integration means exceeds a value, or by AND of the first and second conditions. The same is true of the reset means and the protection means of the second aspect described below.
According to the second aspect of the present invention, there is provided a system interconnection protective device for a private generator set, which interconnects with a system power supply via a circuit breaker, the device comprising frequency detection means for detecting a frequency from an output of the private generator set, frequency change rate detection means for detecting a frequency change rate of the frequency detected by the frequency detection means, voltage variation reference determination means for defining a relationship between the frequency change rate and voltage variation reference by a function, the voltage variation reference increasing a leading reactive power of the private generator set when the frequency change rate detected by the frequency change rate detection means is positive increasing a lagging reactive power of the private generator set when the frequency change rate is negative, and for allowing a gain or a shape of the function to be switched to a plurality of stages, voltage variation reference selection means for setting a plurality of threshold values to the frequency change rate detected by the frequency change rate detection means and issuing a select command for selecting the gain of the function or the shape thereof to the voltage variation reference determination means whenever the frequency change rate exceeds each of the threshold values, frequency change rate integration means for integrating the frequency change rate detected by the frequency change rate detection means, frequency variation tendency determination means for determining whether the frequency rises or lowers, based on a frequency change rate integration amount integrated by the frequency change rate integration means, bias means for applying a bias to the voltage variation reference output from the voltage variation reference determination means to decrease the voltage variation reference when the frequency variation tendency determination means determines that the frequency rises and applying a bias to the voltage variation reference output therefrom to increase the voltage variation reference when the frequency variation tendency determination means determines that the frequency lowers, switch means for determining whether the bias means applies the bias to the voltage variation reference in response to the select command output from the voltage variation reference selection means, and protection means for detecting a frequency variation promoted due to a voltage variation of the private generator set caused by adding the voltage variation reference to which the bias is applied by the bias means, to voltage reference of an automatic voltage regulator of the private generator set, thereby opening the circuit breaker and disconnecting the private generator set from a system bus.
According to the first aspect described above, a frequency change rate of a private generator set is detected. A command for dropping a voltage is issued when the detected frequency change rate is positive, and a command for raising a voltage is issued when the frequency change rate is negative. The output voltage of the private generator set is varied by supplying an automatic voltage regulator with the voltage variation reference obtained by assisting the commands based on a rough tendency of frequency variations.
Thus, a sole operation of the private generator set, which interconnects with a system, can reliably and easily be detected and protected on the side of the private generator set by enlarging the frequency variations and without using any expensive transfer breaker.
According to the second aspect described above, the multi-step detection method reduces an influence upon a system with which the private generator set is interconnecting and enlarges a frequency variation in a sole operation of the private generator set by assisting voltage variation reference, thus detecting a frequency and an abnormal frequency change rate.
The sole operation of the private generator set that interconnects with a system can be detected and protected more reliably and easily on the side of the private generator set by enlarging the frequency variation and without providing any expensive transfer breaker.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.